1. Field of the invention
This invention relates to a knee prosthesis for fitting to a patient as a replacement knee joint.
2. Related Art
Modern total knee replacement involves the resurfacing of the femoral condyles with a metallic component, roughly approximating to the shape of the anatomical femoral condyles, and resurfacing the tibial plateau with a polyethylene component having a metallic base plate. Conformity between the polyethylene of the tibial component and the metallic femoral component has historically been a troublesome area. Ideally the femoral component should be congruent with the top of the tibial component through all, or at least a significant proportion of the flexion of the knee. In knee replacements where the posterior cruciate ligament (PCL) is removed, various cam and post arrangements have been proposed to reproduce the stabilisation effect of the PCL, but these have not always proved effective over a normal range of knee flexion.
U.S. Pat. No. 4,298,992 discloses a knee prosthesis in which during flexion of the knee there is a camming action between a cam follower of the femoral prosthesis component and a cam surface of a post of the tibial prosthesis component. However it was found with this arrangement that with increasing flexion the contact point between the cam follower and the cam surface rises up the cam surface. This has two undesirable effects. Firstly this can lead to wear and deformation at the tip of the post, and secondly the cam follower can jump the post of the patient happens to achieve high flexion.
U.S. Pat. No. 6,123,729 also disclosures a knee prosthesis in which a cam surface engages an articular surface on a post during knee flexion. Whilst this represents an improvement over the prosthesis of U.S. Pat. No. 4,298,992, it suffers from there being a variable contact position of the cam surface on the post. This contact point moves from a position approximately half-way up the post at 90° of flexion, and then as flexion increases, firstly moves down the post and then up again at high flexion. A problem is that the main condyle-meniscal articulation with this prosthesis is incongruent, offering virtually no resistance to antero-posterior subluxation over the range of flexion 30° to 90°. Even at 60° of flexion, if the femur subluxes forward on the tibial component, then the cam surface/post contact point will be much higher up the post than for normal flexing.